Effect of obstacles

We've seen that microwaves used by Wireless LANs propagates along straight lines from the transmitter to the receiver. This is the ideal case, approached in real outdoor environments when there is a clear line of site between transmitter and receiver. However, in real indoor environments, air and distance is not the only "obstacle" encountered by WLAN transmissions. Typical obstacles such as windows, doors, walls, desks, and even people will heavily attenuate WLAN transmission signal, but it can still be string enough to maintain a link on tenths of meters, with only a near line-of-site or even without line of sight at all. This is due to the properties of that radio waves that are reflected, diffracted, or scattered by windows, doors, walls, desks, and even people. Eventually the transmission reaches the receiver, attenuated by the combination of all these effects. This is why predicting losses caused by obstacles is so difficult. For a better understanding, let's have a look at the different effects.

When the transmitted signal is striking a surface it will either be absorbed, reflected, or be a combination of both. This reaction depends on the physical and signal properties. Physical properties are the surfaces geometry, texture and material composition. Signal properties are the arriving incident angle, orientation, and wavelength.

A diffracted wave front is formed when the transmitted signal is obstructed by sharp edges within the path. Secondary waves are formed behind the obstructing body, even though there is no line of site. Indoor environments contain many types of these edges and openings. Thus the resultant diffracted signal is dependent on the geometry of the edge, the spatial orientation, as well as dependent on the impinging signal properties. The result of diffraction of a wave at an obstacle edge is that the wavefront bends around and behind the obstacle edge. Diffraction is best demonstrated by the radio signal being detected around corners and hallways.

If there are many objects in the signal path, and the objects are small relative to the signal wavelength, then the propagated wavefront will break apart into many directions. The resultant signal will scatter in all directions adding to the constructive and destructive interference of the signal.

In indoor environments the signal propagated from the transmitter antenna will experience many different transformations and paths as shown below. At the receiver antenna the resulting signal will be the sum of propagation along along multiple paths. Signals along different paths have traveled different distances and hence reach the receiver with different phases. This will create constructive and destructive interferences.